You definitely want:
Infrared (for nighttime photos)
Ability to take a series of photos at once (sometimes the animal may not be in the right position in the first photo)
A large SD card, minimum 4GB (allows you to stay away from camera longer, thus not leaving human scent)
Long battery life (I change the batteries every 2 months whether they need it or not)
I have a Moultrie Game Spy IR 5.0MegaPixel. I got it at Wal-Mart on sale for $79. I first started using it during the season on a tree near my treestand because I hadn't been seeing anything while hunting. I left it up all season and when the season was over, I moved its location to a spot that was more conducive to turkey hunting. I set out some corn and birdseed after the deer season was over to see what had survived the hunt and I got TONS of photos. In two seperate photos, I got 9 does (3 obviously pregnant) and another shot got 17 hens with 2 jakes trailing them.
I've got mine set up to take 3 pics when movement is detected, and I put an 8GB card in mine. So far I have almost 3000 pics. Once I tweaked the settings to their current settings, I started to get REAL results. Don't be afraid you will be taking too many pics, that's when you get the good ones.
The best photos I have are of a doe/raccoon stand-off, a nice 8-pointer that still had his antlers in late March, a hen stretching her neck to check out the camera, a doe/fawn from a few weeks ago, and some promising bucks that are frequenting my mineral lick rock.
I keep mine up all year. You will get more pics that way and you will see things you didn't even know were there.
USAF F-16 Crew Chief 2001-05
libertas vel mors
"I just want you to know that we are working on (gun control). We have to go through a few processes, but under the radar." Barack Obama to Sarah Brady, 30 March 2011